86 THE ORIGIN OF SPECIES 



doubts as to specific limits augment." He also adds that 

 it is the best known species which present the greatest 

 number of spontaneous varieties and sub-varieties. Thus 

 Quercus robur has twenty-eight varieties, all of which, 

 excepting six, are clustered round three sub-species, 

 namely, Q. pedunculata, sessiliflora, and pubescens. The 

 forms which connect these three sub-species are compara- 

 tively rare; and, as Asa Gray again remarks, if these 

 connecting forms which are now rare were to become 

 wholly extinct, the three sub-species would hold exactly 

 the same relation to each other as do the four or five 

 provisionally admitted species which closely surround the 

 typical Quercus robur. Finally, De Candolle admits that 

 out of the 300 species, which will be enumerated in his 

 Prodromus as belonging to the oak family, at least two- 

 thirds are provisional species, that is, are not known 

 strictly to fulfil the definition above given of a true spe- 

 cies. It should be added that De Candolle no longer 

 J^ believes that species are immutable creations, but con- 

 cludes that the derivative theory is the most natural one, 

 "and the most accordant with the known facts in pale- 

 ontolog}^, geographical botany and zoology, of anatomical 

 structure and classification." 



When a young naturalist commences the study of a 

 group of organisms quite unknown to him, he is at first 

 much perplexed in determining what differences to con- 

 sider as specific, and what as varietal; for he knows 

 nothing of the amount and kind of variation to which 

 the group is subject; and this shows, at least, how very 

 generally there is some variation. But if he confine his 

 attention to one class within one country, he will soon 

 make up his mind how to rank most of the doubtful 



